Using multiple regression analysis, a formula was evolved for estimating fetal weight in utero, based on fetal biparietal diameter (BPD), abdominal diameter (AD) (mean of two orthogonal readings), and femur length (FL), measured by ultrasound within 48 hours before delivery or legal abortion in a stratified sample of 89 pregnancies, approx. 10 in each 500-g weight class up to 5 000 g. Tested on 135 neonates of varying birth weights, the formula evolved, wt = BPD0.972 X AD1.743 X FL0.367 X 10(-2.647), neither under- nor over-estimated weight in any weight class, the error in estimates having a standard deviation of 7.1%, and maximum error being 18% of true weight. To establish an intrauterine growth curve, the formula was applied to 177 longitudinal measurements in 19 normal pregnancies; the estimated weight against gestational age (GA) curve so obtained best fitted a third-degree equation, wt = 1443.4 - 32.32 X GA + 0.203 X GA2 - 0.000215 X GA3 (r2 = 0.978), tallying closely with the birth weight curve obtained in the same population from 4743 pregnancies where gestational age had been assessed by ultrasound in early pregnancy. The present two growth curves, based on fetometry and on birth weight, differ from previous curves used almost universally by pediatricians.